feat(database): add optional build support for SQLite

Miniflux can be build with `go build -tags=sqlite` to test this. Note that
while it builds, it will fail at runtime, as some of the SQL used in miniflux is
postgresql-specific.

internal/database/postgresql.go

@@ -0,0 +1,31 @@
+//go:build !sqlite
+
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package database // import "miniflux.app/v2/internal/database"
+
+import (
+	"database/sql"
+	"time"
+
+	_ "github.com/lib/pq"
+)
+
+// NewConnectionPool configures the database connection pool.
+func NewConnectionPool(dsn string, minConnections, maxConnections int, connectionLifetime time.Duration) (*sql.DB, error) {
+	db, err := sql.Open("postgres", dsn)
+	if err != nil {
+		return nil, err
+	}
+
+	db.SetMaxOpenConns(maxConnections)
+	db.SetMaxIdleConns(minConnections)
+	db.SetConnMaxLifetime(connectionLifetime)
+
+	return db, nil
+}
+
+func getDriverStr() string {
+	return "postgresql"
+}